Wearable displays producing an image in the air are generally divided into two types. That is, there are a helmet-type wearable display worn on a user's head and a glasses-type wearable display. A helmet-type wearable display has a structure in which an optical lens system has an increased volume so as to produce a large image through an expanded field of view (FOV) and is mounted on a user's head, and is thus referred to as a head mounted display (HMD). Therefore, the helmet-type wearable display is used in professional fields requiring a restricted space with little motion, such as military training (cyber flight training) and cyber games.
On the other hand, a glasses-type wearable display, as exemplarily shown in FIGS. 1(a)-1(c) has a lightweight and small-sized structure that is mounted on user's nose and ears like the structure of glasses and is easily used while in motion. Glasses-type wearable displays are divided into three structures.
First, a direct view structure 110 as shown in FIG. 1(a), in which panel and a lens are fixed in front of the eyes, is most classic and basic in design of a virtual image optical system and is formed in a see-closed type in which a user may see an external view. Therefore, the direct view structure 110 needs to recognize an external view in a moving space and is thus disadvantageous.
Second, in order to solve the disadvantage of the direct view structure 110, a top-fixed reflection structure 120 as shown in FIG. 1(b), a top view, in which a panel is fixed at the top and a user may see an external view using a partially reflective surface, is provided. However, since the panel and an optical system group are generally fixed in front of the eyes, the top-fixed reflection structure 120 is difficult to have a thin thickness as in a general glasses structure and a light weight.
Third, a side-fixed light guide structure 130 as shown in FIG. 1(c), a side view, in which a panel and an optical lens group are moved from the front to side frames at the side of eyes and is similar to a glasses lens using a light guide, is provided.
Side-fixed light guide structures are divided into a PBS type 210 and a prism type 220 so as to transmit beams to a pupil through internal reflection of a planar light guide, as exemplarily shown in FIGS. 2(a) and 2(b). Since special partial coating of several tens of layers is performed on respective PBS plane segments and then the segments are bonded so as to uniformly emit beams from respective mirrors and the top-fixed reflection structure 210 is formed of a restricted material, such as glass, the top-fixed reflection structure 210 is difficult to mass produce using a mold structure. On the other hand, the prism type structure 220 may be manufactured using a mold structure and be easily formed of plastic. However, a lens group is disposed distant from the eyes through total internal reflection and thus the two planar light guide types are limited in extension of an FOV and a PBS or prism pattern is virtually visible.
Further, beams are reflected within the effective range of partial PBS mirrors or a prism mirror changing a path to guide the beams to pupils through total internal reflection of a planar light guide and, thus, the conventional methods are limited in the FOV determining the size of an image. As exemplarily shown in FIGS. 3(a) and 3(b), in order to form a range in which beams do not overlap to perform internal reflection, an effective segment partial PBS mirror range 310 or a prism mirror range 320 is restricted in connection with the thickness of the planar light guide. Particularly, since intervals between eyes differ from one person to another and pupils move, an eye motion box (EMB) range to maintain performance of a desired image is considered even in such a change and, thus, there is a severe restriction on increase in the FOV through internal reflection. Moreover, a visual pattern is visible and, thus, the segment partial PBS mirror structure or the prism mirror structure needs to be considered in design.